1. Field of Invention
This invention relates to a converter for converting a reactive power to a corresponding DC signal.
2. Description of Prior Art
The effective power in an AC circuit is the product of a voltage and a current which is multiplied by the cosine of th phase difference between the voltage and the current. The reactive power in the AC circuit is the product of the voltage and the current which is multiplied by the sine of the phase difference. To provide a reactive power to DC signal converter using an effective power converter for measuring the reactive power in the AC circuit, a capacitor is inserted in a voltage or current path in the circuit of the effective power converter for shifting the phase of either the voltage or the current by .pi./2, in order to multiply the phase difference between the voltage and the current by the sine value.
One conventional example of the reactive power to DC signal converter in which the phase is shifted .pi./2 by the capcitor is illustrated in FIG. 2. In that FIG. 2, V sin.omega.t is a voltage input in an AC circuit and A sin(.omega.t+.theta.) is a current input, wherein .omega. is the angular frequency and .omega. is the phase difference between the voltage V and the current A. A capacitor C1, for shifting the phase by .pi./2, is connected across an operational amplifier A1 to which the voltage input Vsin.omega.t is applied. Thus, the operational amplifier al produces at its output a voltage expressed by the following: EQU (V/.omega.)sin(.omega.t+.pi./2) (1)
wherein the input voltage V is shifted in phase by .pi./2. The output voltage is then applied to one input terminal of a multiplier SW. The current input Asin (.omega.t+.theta.) is applied directly to the other input terminal of the multiplier SW. The multiplier SW multiplies the current input A by the voltage output from the operational amplifier A1. As a result, the multiplier SW generates an output EQU (V.multidot.A.#).multidot.sin .theta. (2)
Equation 2 is indicative of a reactive power in the AC circuit. A DC signal corresponding to the reactive power can be produced by smoothing out the reactive power.
The reactive power to DC signal converter shown in FIG. 2 comprises electronic components entirely and is small and highly accurate in operation. The converter is widely used as a reactive power meter in place of an ordinary wattmeter of the electrodynamometer type. In the converter shown in FIG. 2, the phase is shifted .pi./2 by the capacitor Cl. However, the phase shifting with the capacitor essentially varies dependent on the angular frequency (.omega.C). Thus, the term (1/.omega.) is present in the above equation 2, and furthermore, the converter shown in FIG. 2 is limited to a particular frquency designated by the user. This is disadvantageous in terms of cost since different converters must be tailor made to the specified frequency of the users. Accordingly, the manufacturing efficiency is correspondingly low and costs high. The situation is becoming more acute since converter users are increasingly becoming spread throughout the world where frequencies differ from place to place.